This invention relates to a system for detecting, in an absolute value, a rotational position (angle) of a rotary shaft of a motor or other rotary members or a linear position of a work table, piston rod or other linearly displacing members.
A prior art rotary encoder can detect only an absolute position within a single rotation. For detecting an absolute position over a number of rotations, a separate rotation number detection means is provided and the rotation number detected by this detection means is combined with the absolute number within the single rotation. The prior art rotation detection means requires gears of a fairly large reduction ratio for detecting the rotation number in an absolute value. For instance, in a case where a reduction gear mechanism is constructed so as to be capable of detecting 32 rotations of a main shaft, it is conceivable that 32 teeth are provided on the main shaft and 1024 teeth are provided on a reduction output shaft. In this case, an expected accuracy of response at a change point of the rotation number is only an accuracy of one thirty-seconds of one rotation, i.e., about 11.degree. in terms of angle. As will be seen from this example, no high accuracy of detection can be expected notwithstanding that a gear mechanism of a high precision is required. For avoiding such disadvantage, it has also been practiced to obtain the rotation number by counting an incremental pulse. This method, however, is disadvantageous in that the rotation number becomes undetectable for stoppage of an electrical current or other accidental reasons.
It is, therefore, an object of the present invention to provide a system capable of detecting an absolute position over a wide range. It is another object of the invention to perform such absolute position detection with a high precision and without requiring a bulky reduction gear mechanism.